The cleanliness of electronic circuit assemblies (ECA), such as printed circuit boards (PCB) or printed wiring boards (PWB), is generally regarded as being critical to their functional reliability. Ionic and nonionic contamination on circuit assemblies is believed to contribute to premature failures of the circuit assemblies by allowing short circuits to develop.
In the manufacture of electronic circuit assemblies, ionic and nonionic contamination can accumulate after one or more steps of the process. Circuit assembly materials are plated, etched, handled by operators in assembly, coated with corrosive or potentially corrosive fluxes and finally soldered.
In the fabrication of electronic circuit assemblies, e.g., printed circuit boards, soldering fluxes are first applied to the substrate board material to ensure firm, uniform bonding of the solder. These soldering fluxes fall into two broad categories: rosin and non-rosin, or water soluble, fluxes. The rosin fluxes, which are generally only moderately corrosive and have a much longer history of use, are still widely used throughout the electronics industry. The water soluble fluxes, which are a more recent development, are being used increasingly in consumer products applications. However, the water soluble fluxes contain strong acids and/or amine hydrohalides, such fluxes are very corrosive. Unfortunately, residues of any flux can cause circuit failure if residual traces of the material are not carefully removed from the boards following soldering.
While water soluble fluxes can be easily removed with warm, soapy water, the removal of rosin flux from printed circuit boards is more difficult and has therefore traditionally been carried out with the use of chlorinated hydrocarbon solvents such as 1,1,1,-trichlorethane, trichloroethylene, trichloromonofluoromethane, methylene chloride, trichlorotrifluoroethane (CFC113), tetrachlorodifluoroethane (CFC112) or mixtures or azeotropes of these and/or other solvents. These solvents are undesirable, however, because they are toxic and when released into the environment deplete the ozone layer and/or contribute to the greenhouse global warming effect. Thus, use of such solvents is subject to close scrutiny by the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA) and stringent containment equipment must be used. Moreover, if released into the environment these solvents are not readily biodegradable and are thus hazardous for long periods of time.
In view of the toxicity of such solvents and, in view of recent concerns regarding the release of chlorofluorocarbons into the environment, the use of such solvents is being heavily regulated and phased out such that alternatives to such solvents are in immediate need.
Alkaline cleaning compounds known as the alkanolamines, usually in the form of monoethanolamine, have been used for rosin flux removal as an alternative to the toxic chlorinated hydrocarbon solvents. These high pH compounds (e.g., about 12 pH), chemically react with rosin flux to form a rosin soap through the process of saponification. Other organic substances such as surfactants or alcohol derivatives may be added to these alkaline cleaning compounds to facilitate the removal of such rosin soap. Unfortunately, these high pH compounds, like the water soluble soldering fluxes, have a tendency to cause corrosion on the surfaces and interfaces of printed wiring boards if they are not completely and rapidly removed during the fabrication process. Additionally, these cleaning compounds provide high levels of organics to the wash bath. Thus, the water effluents obtained during the cleaning process must be treated to bring the COD and BOD to acceptable levels before disposal.
In other approaches, Daley et al., U.S. Pat. No. 4,635,666 utilize a highly caustic solution having a pH of 13 in a batch cleaning process. This method severely oxidizes the solder applied to the circuit board. In Hayes et al., U.S. Pat. Nos. 4,640,719 and 4,740,247 rosin soldering flux and other residues are removed from electronic assemblies by means of terpene compounds in combination with terpene emulsifying surfactants by rinsing in water.
The complete removal of adhesive and other residues also poses a problem. During the manufacture of electronic circuit assemblies the components are mounted on the upper surface of the board with leads protruding downwardly through holes in the board and are secured to the bottom surface of the board by means of an adhesive. Further, it is sometimes necessary to temporarily protect certain portions of the board from processing steps such as the process of creating corrosion resistant gold connecting tabs at the board edges. This transient protection of portions of the circuit board can be achieved by the application of special adhesive tape to susceptible areas. Once such protection is no longer needed, the adhesive tape must be removed. In both instances, a residue of adhesive generally remains which, if not thoroughly removed, can cause premature board failure. Removal of this adhesive residue has traditionally been carried out by the use of chlorinated solvents which, as already described, are toxic and environmentally undesirable.
Thus, the residual contaminants which are likely to be found on electronic circuit assemblies and which can be removed by the compositions and method of the present invention include, but are not limited to, for example, rosin flux, photoresist, solder masks, adhesives, machine oils, greases, silicones, lanolin, mold release, polyglycols and plasticizers.
In copending, commonly assigned U.S. Ser. No. 731,512, filed Jul. 17, 1991, an improved cleaning composition characterized by non-corrosiveness and low environmental impact, unlike the prior art chlorinated hydrocarbon solvents and alkaline cleaners, are employed for cleaning printed wire board and printed circuit boards. As disclosed therein, the cleaning compositions comprise alkali metal carbonate and bicarbonate salts so combined that during use at the desired concentration of these salts, the pH of the wash solution ranges from about 10 to 12, typically substantially less than 12 and even less than pH 11. Such cleaners are very effective in removing all the flux and other residues from the circuit and wiring boards. Moreover, since the carbonate and bicarbonate salts are nontoxic and are very compatible with the environment, the use of the cleaning compositions disclosed in the above-mentioned application represent a substantial improvement in the art.
The marketplace has become readily accustomed to and therefor have demanded that the above-described cleaning composition and other cleaning compositions be provided in the form of aqueous concentrates which are added to the wash water during the cleaning process. Unfortunately, by providing cleaning compositions in the form of aqueous concentrates, organic stabilizers are required to maintain certain adjuvants such as anticorrosion agents and surfactants in aqueous solution in the concentrate during handling and storage and the like. These additional organics in the aqueous concentrate add to the BOD and COD levels of the water effluents from the washing process. It would be very advantageous to remove this minimal although still present environmental burden from these aqueous cleaning compositions used for cleaning the electronic circuit assemblies as described above.